Generations of craters testify to the erosive power of wind and water

In this colour image, acquired using the High Resolution Stereo Camera (HRSC), operated on board Mars Express by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), a collection of several craters of varying ages and at different stages of erosion can be seen. One very large impact crater some 70 kilometres in diameter, with a fairly steep, towering crater rim, dominates the left (southern) half of the image. A large field of black dunes has formed on the crater floor. The dark material in these dunes probably originates from the erosion of basalt, a dark volcanic rock, and was carried there by the wind. This image was created using data acquired by the high-resolution nadir channel and the colour channels of HRSC. North is to the right.

Perspective views of the surface of Mars can be derived using digital terrain models computed from the HRSC image data. This image shows the view southwards across the field of dark dunes that is located in the largest impact crater, which is some 70 kilometres in diameter. The dark material in these dunes is thought to have originated from the erosion of dark volcanic rock such as basalt and was carried there by the wind.

The nadir channel, which is directed vertically down onto the surface of Mars, and one of the four stereo channels in the DLR-operated HRSC camera system, can be used to create anaglyph images, which produce a realistic, three-dimensional view of the landscape when viewed with red/blue or red/green glasses. In this image, a collection of several craters of varying ages and at various stages of erosion can be seen. In the three-dimensional view, it is easy to see how the profile of the crater is increasingly smoothed off the longer it is exposed to erosion. One very large impact crater, some 70 kilometres in diameter and with a fairly steep, towering crater rim, dominates the left (southern) half of the image.

Digital terrain models can be derived from the HRSC imaging system’s stereo image data; they allocate an altitude value to each pixel on the surface of Mars. The reference level for the altitude values is an ‘areoid’, a therorectical plane of equal gravitational attraction that corresponds to sea level on Earth. The distribution of altitude values can be determined using the colour legend in the upper right of the image. This enables the observer to distinguish the various crater depths.

The Arabia Terra region to the east of the large Ares Vallis outflow valley forms part of the transition zone between the northern lowlands and the southern highlands. The craters described in this article are located in the smaller of the two rectangles highlighted in white. The region is located some 250 kilometres east of Mawrth Vallis, a valley in which clay minerals have been identified on both sides and in the wider environment by the OMEGA spectrometer on board Mars Express. This is a typical product of the erosion of basaltic, volcanic rock due to the effects of water.

Credit:
NASA/JPL (MOLA); FU Berlin.

On our neighbouring planet Mars, it is mainly wind – through its force and the dust and sand particles it carries – that shapes the terrain structures, wearing them away over the course of millions of years. In the early history of the Red Planet, flowing water also contributed to this. It can be seen, for example, in the numerous craters with varied degrees of erosion in the Arabia Terra region, represented in these images acquired using the High Resolution Stereo Camera (HRSC), operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board the European Mars Express spacecraft.

The region is located some 250 kilometres east of Mawrth Vallis, a valley in which clay minerals have been identified on both sides and in the wider environment by the OMEGA spectrometer on board Mars Express. These are typically produced by the erosion of basaltic, volcanic rock due to the effects of water. Arabia Terra forms the transition zone between the northern lowlands and the southern highlands. In a joint project, DLR, the Free University of Berlin (FU Berlin) and the University of Hanover have produced contiguous image mosaics based on HRSC image data, and have recently used the MC-11 (East) map sheet to compute an 1800 by 1300 kilometre topographical image map of Arabia Terra. There are also deposits of clay minerals in the wider environment, which makes this a preferred region for future rover missions.

The images published here show a collection of multiple craters of varying ages and at various stages of erosion. One very large impact crater, some 70 kilometres in diameter and with a fairly steep, towering crater rim, dominates the left (southern) half of the image. A large field of black dunes has built up on the crater floor. The dark material in these dunes is likely to have originated from the erosion of dark volcanic rock such as basalt, which has been carried there by the wind.

An interesting region for future lander missions

On the upper right edge of the crater (images 1, 2 and 4), three structures can be seen that cut into the flank and are reminiscent of sapping valleys. These form when ground water escapes under the edge of the terrain. The caverns created by this collapse and the eroded material is transported away along the course of the valley by the flowing water. At the lower edge of the larger valley at its outflow, there is a three by four kilometre fan-shaped deposit; this is not easy to see and is most clearly visible in the colour-coded topographical overview map (image 4). It is reminiscent of the sediments in a river delta and most probably consists of eroded material that has been transported out of the three small valleys by water.

Directly to the right near the large impact crater is a smaller, very heavily eroded crater, the interior of which has been filled with sediments. Despite this, several traces of the crater rim can still be observed. Further to the north (right in images 1, 2 and 4) is a similarly large impact crater approximately 30 kilometres in diameter, which has been completely filled with sediments and harbours a black dune. Its crater rim has almost completely disappeared and its circular outline can only be seen in the topographical image map (image 4).

A river as wide as the Mississippi

To the right, below this crater, is a valley with a shape very similar to dendritic river courses on the Earth. Such valleys that wind through the landscape are called ‘meanders’ after a river in Asia Minor. Its dendritic, snaking course is reminiscent of rivers on Earth. The section of the valley that can be seen in these images varies between one and a half and four kilometres in width, and so is comparable with the Mississippi. The river runs 500 kilometres northwards and flows out into Acidalia Planitia, a plain that forms part of the northern lowlands of Mars.

This region of the 'old' Mars highlands shows numerous traces of the eventful history of Mars. In it, many different developments and changes to the surface of Mars that took place in the early history of the planet can be very clearly traced. Rivers crossed the landscape and must have transported enormous volumes of water northwards periodically. Impact craters have been filled with sediments, and their crater rims and ejecta blankets have been worn away by the power of the water and the persistent wind through the ages.

Image processing

The images from the High Resolution Stereo Camera (HRSC) were acquired on 19 November 2014 during Mars Express orbit 13,728 at longitude 349 degrees east and latitude 25 degrees north. The image resolution is approximately 20 metres per pixel. The colour image (image 1) was acquired using the nadir channel of the HRSC, which is directed vertically down onto the surface of Mars; the perspective oblique view (image 3) was computed from the HRSC stereo channels. The anaglyph image (image 2), which creates a three-dimensional impression of the landscape when viewed with red/blue or red/green glasses, was derived from the nadir channel and one stereo channel. The colour-coded aerial view (image 4) is based on a digital terrain model of the region, from which the topography of the landscape can be derived.

The HRSC experiment

The HRSC was developed at DLR and built in collaboration with partners in industry (EADS Astrium, Lewicki Microelectronic GmbH and Jena-Optronik GmbH). The science team, which is headed by Principal Investigator (PI) Ralf Jaumann, consists of 52 co-investigators from 34 institutions in 11 countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof.